The present invention relates to a polymer electrolyte composite for driving an electrolytic capacitor, an electrolytic capacitor using the same, and a method of making the electrolytic capacitor.
An electrolytic capacitor has such a structure that a cathode and an anode having a dielectric oxide film formed thereon by anodic oxidation sandwich an electrolyte. Kinds of electrolytes can be categorized into three types: ion conductive type using a liquid electrolyte; ion conductive type using a solid electrolyte; and electron conductive type using a solid electrolyte.
An electrolyte of ion conductive type using a liquid electrolyte has an advantage in that a high voltage for anodic oxidation can be applied between the anode and the cathode of the electrolytic capacitor when such a high voltage application is needed to reform or recover the dielectric oxide film having deep defects or unformed places. Namely, it is capable to apply a high forming voltage for the purpose of reforming the dielectric oxide film, thereby to be able to easily recover defects therein. Here, the capability of application of a high reforming voltage means that sparks are not generated at lower voltages voltages, i.e. that sparking voltage or minimum voltage for spark generation is high.
However, such an electrolyte of ion conductive type using a liquid electrolyte has drawbacks as well. In order to retain a liquid electrolyte between an anode and a cathode, a separator is required, which should be able to be sufficiently filled with the liquid electrolyte and which sufficiently separates the anode and the cathode. In order to meet the requirement, it becomes necessary to use a separator material, such as paper and nonwoven fabric, having a sufficiently high weighting, i.e. product of density and thickness. Although a liquid electrolyte itself has a comparatively high ionic conductivity and a comparatively low electric resistance which is usually expressed by equivalent series resistance (ESR), the combined body of the separator and the liquid electrolyte gets to have a high electric resistance because of the high weighting of the separator. In other words, the total resistance loss of a resultant electrolytic capacitor becomes large due to the separator although the resistance loss by the liquid electrolyte itself can be made comparatively small. Furthermore, since the liquid electrolyte is of liquid by definition, it has drawbacks as to e.g. liquid leakage and difficulties in mounting to electrical devices and machining.
For the above reasons, it has been studied to solidify liquid electrolytes. Solid electrolytes do not have drawbacks caused by the use of liquid. In one example of a solid electrolyte having an electronic conductivity, polypyrrole is used as electrolyte in place of the liquid electrolyte, and a porous resin such as polypropylene and polyethylene is used as separator. Since it uses electronic conductivity, it has a low electric resistance, contributing to a small resistance loss of the resultant electrolytic capacitors. However, in the case of the solid electrolytes of electron conductive type, it is difficult to sufficiently recover the dielectric oxide film by applying a high reforming voltage even though the dielectric oxide film has defects therein. This is because in the case of such type, sparks are likely to be generated with an application of a comparatively low reforming voltage. Namely, such solid electrolytes of electron conductive type have little function of recovering the dielectric oxide film.
On the other hand, solid electrolytes of ion conductive type can be categorized into inorganic systems and polymer systems. The inorganic systems have drawbacks in that they are heavy, inflexible and difficult to form, although they have an advantage in their high ionic conductivities.
Solid polymer systems, i.e. ion conductive polymers, are drawing attention because they are superior in their mechanical properties in e.g. lightness in weight, flexibility and formability or easiness of manufacturing, although they have much lower ionic conductivities.
Examples of ion conductive polymers having been reported are: mixture of polyethylene oxide (PEO) with a lithium salt, having an ionic conductivity of about 10−4 S/cm at 100° C. (see “Polymer”, 14, 586 (1973)); mixture of diisocyanate crosslinked polymer of triol type polyethylene oxide with a metal salt, having an ionic conductivity of 10−5 S/cm at 30° C. (See Japanese Laid-open Patent Publication Sho 62-48716); copolymer of oligooxyethylene polymethacrylate and an alkaline metal salt of methacrylic acid in which pair ions are fixed, having an ionic conductivity of 10−7 S/cm at room temperature (see “Polymer Reprints Japan”, 35, 583 (1986)); and mixture of monofunctional and multifunctional acryloyl-modified polyalkylene oxides with an alkaline metal and/or an alkaline earth metal, having an ionic conductivity of 10−3 S/cm at 20° C. (See Japanese Laid-open Patent Publication Hei 8-295711). Further, an example of ion conductive polymers having been actually used for electrolytic capacitors is composed of a solvent and an electrolyte salt together with a thermally metamorphosed polymer and/or a cellulose derivative, having an ionic conductivity of 10−3 S/cm at room temperature (See Japanese Laid-open Patent Publication Hei 5-55088), wherein the solvent contains a polyhydric alcohol compound having a molecular weight of 200 or lower, and the thermally metamorphosed polymer contains albumen protein and/or β-1,3 glucan.
However, some of the above described ion conductive polymers have too low ionic conductivities to cause too large resistance losses, so that electrolytic capacitors using such ion conductive polymers cannot have sufficient performances. Other ones of the above described ion conductive polymers have too low heat resistances, though they may have ionic conductivities similar to those of liquid electrolytes. Furthermore, the existence of metal salts in the above described ion conductive polymers is likely to cause short-circuiting of the resultant electrolytic capacitors when they are used at a high temperature, and makes it difficult to obtain satisfactory characteristics.